Intermolecular interactions in mixtures of 1-n-butyl-3-methylimidazolium acetate and water: Insights from IR, Raman, NMR spectroscopy and quantum chemistry calculations

The intermolecular interactions in 1-n-butyl-3-methylimidazolium acetate (BmimAc)/water mixture were studied by examining the mole fraction dependence of the vibrational modes measured with infrared and Raman spectroscopy, as well as of the 1H, 13C, 15N and 17O NMR chemical shifts. Quantum chemistry calculations were carried out to assign as clearly as possible the vibrational modes of the cation and anion as well as to get information on the expected effect of the mixture composition on the vibrational modes and charge distribution in BmimAc. The behavior of the obtained spectroscopic data is consistent with the physical picture that when water is added to BmimAc, the interactions between cation–anion are weakened in favor of those between water and anion. The clear spectral signature is associated with the strong blue/red shift of the stretching and bending modes of the C(2)–H, respectively. Consistently with this spectral behavior, the corresponding 1H and 13C chemical shifts decrease when adding water to BmimAc. The strengthening of the water–anion interactions is also traced back to both the blue shift of the νs(COO−) at around 1380cm−1 and ν(C–C) of acetate at around 920cm−1 as well as the blue shift of the δ(COO−) at around 637cm−1 and ω(COO−) at 615cm−1. The 1H, 13C and 17O chemical shifts of COO− are consistent with the corresponding spectral behavior. •The intermolecular interactions in BmimAc/water mixture were studied with IR, Raman, and multinuclear NMR spectroscopy.•DFT calculations helped to clearly assign the BmimAc vibrational modes and to interpret water induced spectral changes.•The results point to a picture when added water weakens the interactions 'cation-anion' in favor of those 'water and anion'.